Field of the Invention
The present invention relates to an image display apparatus and a control method thereof.
Description of the Related Art
A liquid crystal display has a liquid crystal panel and a backlight unit. The liquid crystal panel has a plurality of liquid crystal elements. The transmittance of each liquid crystal element is controlled by controlling the voltage applied to each liquid crystal element. The voltage applied to each liquid crystal element is determined in accordance with gradation values of liquid crystal elements input to the liquid crystal panel. The backlight unit is provided on the backside of the liquid crystal panel. Light from the backlight unit is modulated as it passes through each liquid crystal element. An image is displayed on the screen by the modulated light (light that has passed through each liquid crystal element).
Sometimes, however, the orientation of liquid crystal molecules in the liquid crystal element may be disturbed for various reasons. Such disturbed orientation of liquid crystal molecules may sometimes lead to unevenness in the transmittance of the liquid crystal panel. For example, the liquid crystal panel maybe subjected to stress via a support member that supports the liquid crystal panel. Such stress applied to the liquid crystal panel may cause a disturbance in the orientation of the liquid crystal molecules, unevenness in the transmittance, and the like. Unevenness in the transmittance appears as non-uniformity (unevenness) in brightness or chromaticity (visual non-uniformity) and hinders visual perception of an image. The intensity of such visual non-uniformity is dependent on the voltage applied to each liquid crystal element. For example, visual non-uniformity occurs to a greater degree in a case where the voltage applied to each liquid crystal element is low. The visual non-uniformity is particularly pronounced in a case where a black image (entirely black image) is displayed in an IPS normally black mode. Such visual non-uniformity will be hereinafter referred to as “black non-uniformity”.
Shapes and sizes of patches that cause black non-uniformity are not uniform and change because of various factors. Black non-uniformity may vary because of the impact applied to the liquid crystal display, for example, or may vary due to changes in the ambient conditions such as temperature. Therefore, an image processing method that is based on the measurement results of the status of black non-uniformity at a certain time point may not be effective in reducing the black non-uniformity.
As a method of reducing black non-uniformity caused by various factors, a method whereby voltage applied to liquid crystal elements is increased, i.e., the gradation value input to the liquid crystal panel is increased, has been proposed (Japanese Patent Application Laid-open No. H10-84551).
Also, various methods called local dimming have been proposed as a method of enhancing the contrast of an image displayed on a liquid crystal display (displayed image). For example, a technique relating to local dimming is disclosed in Japanese Patent Application Laid-open No. 2002-99250. In the technique disclosed in Japanese Patent Application Laid-open No. 2002-99250, a plurality of light sources are used as the light source of the backlight unit. The emission brightness of the plurality of light sources is controlled individually based on the brightness of the image data, and the image data is corrected based on the emission brightness of each light source. The contrast of the displayed image can be enhanced with the use of such a technique.